Check if mouse position collides with the non-transparent shape of an image

pygame-mask-mouse.py

import sys
import pygame

pygame.init()

screen = pygame.display.set_mode((800, 600))
clock = pygame.time.Clock()


class Triangle(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        
        # alternatively just use pygame.image.load(...).convert_alpha()
        self.image = pygame.Surface((300, 250), pygame.SRCALPHA)
        self.active = False
        self.redraw((255, 0, 0))

        self.rect = self.image.get_rect(center=(400, 300))  # screen center
        self.mask = pygame.mask.from_surface(self.image)

    def redraw(self, color):
        # this just draws the image. you don't need it if you got a static image.
        self.image.fill((0, 0, 0, 0))
        w, h = self.image.get_size()
        pygame.draw.polygon(self.image, color, [
            (w // 2, 0),
            (w, h),
            (0, h)
        ])
        self.mask = pygame.mask.from_surface(self.image)


triangle = Triangle()

while True:
    clock.tick(120)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()

    screen.fill((0, 0, 0))

    screen.blit(triangle.image, triangle.rect)

    x, y = pygame.mouse.get_pos()

    # get the position of the mouse within the mask, could be negative
    pos_in_mask = x - triangle.rect.x, y - triangle.rect.y

    # check if colliding with image, and get the mask value at the position.
    if triangle.rect.collidepoint(x, y) and triangle.mask.get_at(pos_in_mask):
        if not triangle.active:
            triangle.redraw((0, 255, 0))
            triangle.active = True
    else:
        if triangle.active:
            triangle.redraw((255, 0, 0))
            triangle.active = False

    pygame.display.flip()